Digital-to-Analog converters (DACs) are found in many electronic devices. For example, DACs are used in PDAs (Personal Digital Assistant), cellular phones, computers, video players and CD players. DACs convert a digital signal into an analog signal. Analog signals include music and voice.
Cell phones, for example, may transmit analog signals. In order to transmit an analog signal, a mixer is used to convert a signal from one frequency to a higher frequency. For example, an analog signal, such as music or voice (baseband analog signal), may be up-converted to a higher frequency for ease of transmission. A mixer is often used to up-convert analog signals for transmission.
Various types of mixers may be used such as active and passive mixers. Active mixers usually have an associated gain where as passive mixers may have a gain of one at best. Active mixers include single-ended and differential implementations. Differential mixer implementations are often used to cancel undesired noise. A differential mixer may be implemented in a single-balanced configuration or a double-balanced configuration for example.
Noise may be added to a signal being transmitted. Noise may be any electrical contribution added to a signal that was not part of the original source that created the signal. Noise may be added to a signal during digital-to-analog conversion or during the up-conversion of a signal during mixing. During the process of converting a digital signal to an analog signal, the original signal may be distorted. There are many types of distortion such as harmonic distortion and intermodulation distortion.
Low-pass filters are often used to remove high frequency noise that may be created by a DAC. The high frequency noise created by a DAC is usually removed by a low-pass filter before an analog signal from the DAC is up-converted by a mixer for transmission. For example, GmC filters or op-amp RC biquadratic filters are often used to remove high frequency noise from a signal before up-converting the signal with a mixer.
Because DACs and mixers are often used on electronics that are portable, the amount of area used to implement them is an important design consideration. The area used to implement a circuit is also proportional to the cost of implementation, especially for integrated circuits where a smaller silicon area corresponds to a cheaper integrated circuit. Portable electronics that use DACs and mixers also have low power requirements. As a result, in many applications DACs and mixers should be small physically and use as little power as possible.